1. Field of the Invention
This invention is in the field of piezoelectric transducers for ultrasound devices, more particularly, piezoelectric transducers comprising piezoelectric cylinders isolated from a support matrix by a gas or vacuum and arranged such that they are separated from each other by less than one wavelength in that matrix.
2. Description of Related Art
Transducers are devices that transform input signals into output signals of a different form. In ultrasound devices, they transform signals of electrical energy into acoustic energy or produce electrical signals from absorbed sound waves. Piezoelectric ceramic materials are particularly effective for this type of electromechanical energy conversion and have found wide use in the transducer field. Many piezoelectric ceramics have very high electromechanical coupling coefficients, kT (approximately 0.5), which indicate how effective a material is at transferring electrical energy into mechanical energy.
In the fields of non-destructive testing of materials, bio-medical non-invasive diagnostics, and ultrasonic power generation, it is highly desired that the source (transmitter) of ultrasound, that is, the transducer device, be characterized by high transduction in the medium of transmission. It is further desired that the receiver of ultrasound be very sensitive to detect even the minutest ultrasonic vibrations, irrespective of the medium or the mechanism by which they are generated.
A second important property for effective ultrasound transducers is the acoustic impedance of the transducer material. Acoustic impedance describes the compressibility of a material and is found by taking the product of the density of a material and the velocity of sound in that material. When a sound wave propagating in material X encounters an interface between X and a second material Y, the size of the difference between the acoustic impedances of X and Y determines the amount of sound energy that is transmitted across the interface and the amount of sound energy that is reflected back into the first material. The greater the difference, the less sound energy that is received into the second material. The transmission of sound energy between two materials is termed acoustic coupling, higher coupling means higher transmission of sound energy. The size of the difference between the values of acoustic impedance is what determines the degree of acoustic coupling in that system. Systems with low differences in acoustic impedance exhibit the best coupling. Piezoelectric ceramics, such as Pb(Zr, Ti)O3 (PZT), have very high acoustic impedances (Z), on the order of 107 Rayl (kg/m2.*s), as compared with air, where Z=410 Rayl. In ultrasound applications, the large difference in acoustic impedance between the probe material (e.g., water) and the monolithic piece of ceramic results in a large proportion of reflected sound waves at the transducer surface. Therefore, the information contained in those sound waves about the probed material is lost because it is not received by the transducer efficiently.
One solution to this problem of poor acoustic coupling is to, create matching layers between the monolithic piece of ceramic and the sample and to use a backing medium behind the ceramic. These layers attenuate sound energy and still lose energy to reflection and are not a perfect solution to the problem. A second solution is to combine the strong piezoelectric characteristics of a ceramic with the better acoustic coupling properties of another material in a composite. Most early attempts to create composites involved loading ceramic particles into a polymer matrix to create a homogenous composite. These composites had low acoustic impedances, but the polymer shielded the piezoelectric ceramic particles from applied electric fields, preventing poling of the ceramic particles. In addition, the polymer acted to dampen waves generated by the ceramic.
Efforts to solve these problems resulted in the development of composites consisting of a porous three-dimensional piezoelectric ceramic network, which could be impregnated with a polymer to lower the acoustic impedance of the overall structure. Shrout et al. U.S. Pat. No. 4,330,593 discloses a method for forming a so-called 3-3 structure (3-3 indicates the ceramic is interconnected in all three directions, and the polymer is also interconnected in all three directions). Since their development, it has been realized that the nature of the phase interconnection controls the dielectric flux pattern and mechanical stress distribution in the composite material.
One theoretically promising arrangement of phases taught by Klicker et al. in U.S. Pat. No. 4,412,148 was a polymer matrix connected in three dimensions, impregnated with piezoelectric ceramic rods oriented in the same direction. This design was termed 1-3 connectivity. The theoretical concept was that the polymer matrix was much softer and had better acoustic coupling with water or tissue and would deform when impacted by a sound wave. The polymer would bind to the side surfaces of the piezoelectric ceramic rods and would transfer the strain energy into the ceramic. In this configuration, the many small rods would have a much greater surface area under strain than a monolithic ceramic. It was hoped this would result in more mechanical energy being transferred. While this configuration did not realize its theoretical potential, partially because most polymers used had very high Poisson ratios which generated internal stresses that opposed the applied stress of the sound waves, it was still a tremendous improvement over previous designs in terms of piezoelectric voltages and sensitivity. The lower dielectric permitivity of the polymer allowed for more complete poling of the piezoelectric material. More complete poling, coupled with a lower overall dielectric constant, allowed for higher piezoelectric voltages than in the monolithic ceramic.
While these composites offer improved acoustic coupling and mechanical response, they still have problems. Depending on the arrangement of rods in the matrix, there is the potential for a so-called grating lobe, a form of acoustic noise, to develop during transmission of ultrasonic waves. Grating lobes consist of undesirable ultrasonic waves being emitted in the directions determined by the pitch of the piezoelectric cylinder arrangement, which acts to deteriorate the ultrasound image. Nakaya et al. U.S. Pat. No. 4,658,176 offered a solution to this problem by spacing apart the cylinders at less than one wavelength of the fundamental frequency of the transducer. This arrangement was found to ameliorate the problem of grating lobe formation and improve ultrasound images obtainable with 1-3 composites.
Despite these improvements, performance problems still remain for piezoelectric transducers. The modern piezoelectric composites offer excellent acoustic matching for human tissue and the flexibility needed for medical probes, but they still have acoustic impedances which remain much greater than what is needed for non-contact applications where transmission through air is necessary. Non-contact ultrasound, which is particularly important for materials characterization, requires good acoustic coupling between air and the transducer to achieve high resolution and polymers with acoustic impedance values in excess of 106 Rayl.
An additional challenge in all piezoelectric ceramics is an effect known as planar coupling. In most transducers, the composite is placed between electrodes and polarized in the direction perpendicular to the electrodes, or the 3 direction. The object is to apply an electric field to the composite and cause displacement in the 3 direction, generating ultrasound waves. In most piezoelectric ceramics, such as PZT, when a field is applied in the 3 direction, there is simultaneous mechanical action in the 1 and 2 directions that are perpendicular to the 3 direction. This is known as planar coupling. While reducing the size of the piezoelectric element helps reduce the magnitude of the planar coupling, the problem remains. In 1-3 composites, planar coupling in the piezoelectric cylinders generates vibrations that propagate through the polymer to other elements in the transducer creating noise, which is termed crosstalk in the art. This noise reduces the resolution of the device. This type of noise is especially troublesome in devices where one part of the array of cylinders is used to transmit ultrasound waves and another part is used to receive the reflected waves. In these arrangements, the waves resulting from planar coupling in the transmitting cylinders are propagated through the polymer to the receiving cylinders creating noise and reduce the image quality. Therefore, the object of the present invention is to overcome deficiencies in the prior art.
The current ultrasonic transducer devices utilize a piezoelectric material, the front and back faces of which are bonded with a variety of materials that modify the resonance and frequency characteristics of the piezoelectric material with respect to ultrasound transmission in a given medium. In such devices, the piezoelectric materials used are: Lead Zirconate-Lead Titanate solid solutions, Lead meta Niobates, Lead Titanates, Lead Magnesium Niobate, Lithium Niobate, Zinc Oxide, Quartz, Barium Titanate, polymer-based homogeneous materials, polymer matrix solid piezoelectric materials, etc. Materials used on the back, front, and on the sides of the piezoelectric materials are: rigid, porous, monolithic or composite, particulate, or fibrous metals, alloys, ceramics, polymers, etc. Depending upon the type of piezoelectric material and those that surround it, the devices according to the current art can be made to generate high transduction in the medium of ultrasound transmission. See Bhardwaj U.S. Pat. No. 6,311,573.
If the devices according to the current art are to be used for certain applications, such as for power generation or for high transduction in attenuative media (gases, coarse grained, open or closed cell materials) particularly in high frequency range, say from 100 kHz to greater than 1 MHz, then one has to apply relatively high electrical power to the devices. Whereas some applications can be successfully executed by doing so, yet there are others that cannot. The reason for this being high power excitation of transducers results in the heating of the piezoelectric material, subsequently destroying the entire device. Besides this, too high electrical power can be dangerous and more cumbersome to handle in a practical manner. Therefore, it is necessary to develop a piezoelectric device that is inherently characterized by transduction efficiency higher than those that are produced according to the current art. The present invention has been shown to overcome the limitations of the prior art.